Modern microelectronic devices are commonly produced using a photolithographic process. In this process, a semiconductor wafer is first coated with a layer of a photoresist. This photoresist layer is then exposed to illuminating light using a mask and subsequently developed. After the development, non-exposed photoresist is removed, and the exposed photoresist produces the image of the mask on the wafer. Thereafter, the uppermost layer of the wafer is etched. Thereafter, the remaining photoresist is stripped. For multilayer wafers, the above procedure is then repeated to produce subsequent patterned layers.
Increasing the number of components in microelectronic circuits produced using the above photolithographic process requires the use of very high resolution images in photoresist exposure.
It should be appreciated by those skilled in the art that to produce an operational microelectronic circuit, a mask must be as defect-free as possible, preferably completely defect-free. Therefore, mask inspection tools are needed to detect various defects in the masks that can potentially reduce the microelectronic circuit fabrication yields.
Certain kinds of mask defects (such as extra pattern, missing pattern or particles can be detected by various inspection methods. A well known defect detection technique is known as die to die comparison. Die to die comparison involves comparing an image of a die to an image of another die. Die to die comparison is not effective in single die masks or in non-die areas of a mask.
There is a need to provide systems and methods for defect detection and especially for defect detection of masks.